To ensure a specified operational reliability for large rotatable machinery, such as a turbine-generator used to produce electric power, it is necessary to accurately determine the torsional natural frequencies and/or dynamic response of the turbine-generator over a frequency range of interest. One exemplary range may be from approximately 70 Hz to approximately 140 Hz.
For many turbine-generators, the torsional frequency response of rotatable components thereof, such as a rotatable shaft of the turbine-generator, is considered to be a significant design factor of the machine. For example, excessive levels of torsional vibration at the rotating shaft of the turbine-generator may cause stresses that could damage or break the shaft.
It is known that water jets have been used to provide torsional shaking during operation of the turbine-generator. One noticeable disadvantage of this technique is that shaking is essentially limited to frequencies corresponding to harmonics of the rotational frequency of the rotating structure. This is cumbersome and prone to inaccuracies since shaking performed at a harmonic of the rotating frequency, generally results in torsional vibration signals that tend to have large amounts of noise.
It is also known to use a hydraulic shaker that uses electronically-operated valves to control the flow of high pressure hydraulic fluid to a rotating shaker head. The resulting pulsations of hydraulic fluid on the shaker head can generate a relatively high level of torsional torque. In practice, however, the assignee of the present invention has experienced rather poor reliability with such hydraulic shakers.
For field-testing of a turbine-generator, it is known to use at power plant sites a generally time-consuming and burdensome off-line test that involves creating a short circuit on the generator or high-voltage side of a main step-up transformer. This test involves reconfiguring the turbine-generator and its current protection system in order to apply a negative sequence current to the generator for creating shaking power. This technique is also prone to noise issues since the torsional shaking is limited to harmonics of the rotating frequency. For torsional shaking subject to such a constraint, as explained above, the torsional signals tend to have large amounts of noise, and, consequently, a resulting signal-to-noise ratio may not be conducive for accurately determining the torsional natural frequencies and/or dynamic response of the turbine-generator at low-vibration power.
Accordingly, it would be desirable to provide an improved torsional shaker apparatus for large rotating machinery, such as a turbine-generator, that avoids or reduces the foregoing shortcomings.
It is also desirable to provide a torsional inspecting means that may make use of equipment that already may be part of a power generating system and, at a relatively low cost and without requiring any substantial downtime of a machine undergoing inspection, allows for accurately determining the torsional natural frequencies and/or dynamic response of the machine.